We removed rat fetuses from the uterus without disturbing the placental implantation site, placing the fetuses in the peritoneal cavity either within intact membranes (with or without amniotic fluid) or outside of the fetal membranes. Untouched littermate fetuses served as controls. The surgery was performed on day 17 of gestation and the outcome was analyzed at term, 4 days later. The utero-abdominal pregnancies with intact membranes and amniotic fluid yielded fetuses in all respects indistinguishable from littermate controls. Extrauterine fetuses maintained within their membranes but without amniotic fluid as well as extrauterine and extramembranous fetuses displayed identical features: body weight was slightly reduced; after correction for body weight, only their lungs weighed significantly less and contained less deoxyribonucleic acid. In addition, fetuses in these two groups had multiple joint contractures and occasional webbing of the skin. These experimental findings in rats indicate an important role of amniotic fluid in fetal development, especially of the lung.